1. Field of the Invention
The present invention relates to a fuel-cut control system which is suitably applied to automotive vehicles equipped with both an internal combustion engine with a fuel-cut device and a lock-up type automatic transmission.
2. Description of the Prior Art
An automotive automatic transmission has a torque converter which provides varying the drive ratios between a drive member (pump impeller) and a driven member (turbine runner), thus providing varying amounts of torque increase and provides absorbing torque fluctuations. Such a torque converter experiences relative movement (also called converter slip) between the drive and driven members. Thus, an efficiency of torque transmission in automatic transmissions is generally inferior to that in manual transmissions. In order to solve the above-mentioned problem, recently, there has been proposed and developed a lock-up type automatic transmission equipped with a lock-up torque converter. One such lock-up torque converter has been disclosed in U.S. Pat. No. 4,305,487. The lock-up torque converter employs a lock-up mechanism which is operable for directly and mechanically interconnecting the converter cover and the turbine runner to transmit engine power from the crankshaft directly to the turbine runner in a lock-up piston operating range (generally abbreviated as a "lock-up range"). As is generally known, relatively large torque fluctuations can occur at low speeds such as 40 km/h or less, for example. In contrast, there are less torque fluctuations at medium and high speeds. For the reasons set forth above, the lock-up range is usually set to be substantially equivalent to medium and high speed ranges in which it is unnecessary to multiply torque produced by the power plant (engine) and to absorb torque fluctuations. Owing to the direct connection between the engine crankshaft and the transmission input shaft, fuel economy can be remarkably improved in the lock-up range in which the lock-up piston or clutch is engaged.
In consideration of fuel consumption, many of the above-noted automobile with a lock-up type automatic transmission further include a fuel-cut device so as to stop fuel supply to the engine during coasting of the vehicle, i.e., when the acceleration pedal is released. The fuel-cut device in general functions to perform a fuel-cut operation during coasting and to restart fuel-supply upon the vehicle running state is changed from coasting to acceleration running. The fuel-cut device also operates to restart fuel-supply to the engine in order to prevent the engine from being stopped, in the event that the engine revolution speed becomes less than a predetermined lower limit even during coasting.
In automobiles equipped with a power unit which includes a power plant with a fuel-cut device and a lock-up type automatic transmission in combination, the vehicle experiences rapid increase in output power of the engine owing to re-start of the fuel-supply, at a particular timing wherein the fuel system is transited from the fuel-cut state to the fuel-supply state in response to shift from coasting to acceleration running. In this case, if the lock-up torque converter was conditioned in the lock-up range, the vehicle occupants would feel uncomfortable shock and vibrations owing to a rapid increase in positive torque fluctuations in the torque converter. To avoid this, Japanese Patent First Publication No. 62-106174 proposes and teaches a temporary release of the lock-up clutch even in the lock-up range when the fuel-cut is released, i.e., the fuel-supply is resumed or recovered in response to shifting from the coasting state to the acceleration running state. Such temporary release of the lock-up clutch can effectively attenuate shocks and vibrations during the fuel-recovery operation. The temporary release operation of the lock-up clutch may be traded off to an acceleration performance of the vehicle, and to such a good direct drive feeling as direct connection obtainable in manual transmissions. In other words: the temporary release operation causes the drivability to deteriorate due to torque loss.